An automatic sheet feeding apparatus is used to supply originals or sheets disposed in a stack on a sheet receiving table to a processor such as a reader in a facsimile system or an exposure station of a copying machine by separating the sheets into single sheets. A variety of sheet separating and feeding systems are known to separate and supply sheets from a stack. In one scheme called bottom-fed type, sheets are sequentially fed one by one from the bottom of the stack disposed on the sheet receiving table. An apparatus of such type is provided with a separating mechanism which typically comprises a separating roller having part of its peripheral surface projecting above the sheet receiving surface of the table for contact with a lowermost one of the sheets in the stack, and a separating member, for example, a brake member, which is disposed in abutment against the separating roller. The sheet separation takes place by the different magnitudes of the coefficient of friction between the separating roller and the sheet, between adjacent sheets or between the sheet and the brake member. Even if the stack contains sheets of different qualities such as those varying in thickness, the apparatus is normally capable of separating and feeding them.
However, in facsimile systems and copying machines, originals may be used which are curled, wrinkled, have a very thin or very thick thickness or have photographs applied thereto. When the described separat is used to feed such sheets which may vary widely, jamming may occur at the location of the mechanism. Alternatively, more than one sheet may be fed in superposed relationship, resulting in a failure to feed a single sheet. Accordingly, whenever such sheets must be fed, it is necessary to feed such sheets one by one manually rather than by utilizing an automatic feeding mechanism. Consequently, a sheet passage must be provided to enable a manual feeding, separately from a sheet passage in which the separating mechanism is disposed. This results in an increased size and a complex arrangement of the apparatus.
The brake member and the separating roller typically define a wedge-shaped space therebetween, into which the leading end of a plurality of sheets may advance, whereupon a lowermost one of the sheets which contacts the separating roller is fed alone while upper sheets are stopped by the brake member. The separated sheet is conveyed to a processor, but when that sheet is subject to a conveying effort by the processor, it may drive the brake member in a direction to move it away from the separating roller, thus, reducing the pressure of contact therebetween. If the next lowermost sheet is present in the wedge-shaped space, the reduction in the pressure of contact between the brake member and the separating roller in combination with the friction of the moving lowermost sheet may cause the next lowermost sheet to be fed in following relationship with the lowermost one, thus disavantageously causing an overlapping feed.
The sheets in the stack are adapted to be delivered to the nip between the brake member and the separating roller in a manner such that the leading edge of a lower sheet goes forwardly of the leading edge of another sheet which is located immediately above the first sheet. However, the sheets disposed on the table are subject to a limitation on the height of the stack. In the prior art practice, a lateral sheet guide which guides the lateral edge of the sheets or a sidewall of the table may be provided with a painted line, groove or or a label which indicates the limiting height of the stack. Alternatively, no indication may be provided, and instead a written notice is given on the table or nearby to indicate the permissible height of the stack and the number of sheets in the stack. However, these indications may be difficult to recognize, giving rise to the likelihood that the sheets may be stacked beyond the limit. The provision of a special indication in the form of a painted line, imprint or labelling increases the cost of the apparatus.
It is also to be noted that an apparatus of the type described is often provided with a pair of cross sheet guides between which the sheets of the stack are disposed and guided in the sheet feeding direction so that a skewing of the sheets may be prevented. Sheets being fed may have a variety of sizes, and accordingly the cross sheet guides must be provided in a displaceable manner to accommodate for sheets of varying sizes. In one arrangement which permits a change in the spacing between the cross sheet guides, the pair of cross guides are displaceable in a direction perpendicular to the sheet feeding direction so that the guides are aligned with the lateral edges of the sheets. In another arrangement, the cross sheet guides are brought into registry with a scale provided on the table and indicating various sheet sizes. Because this requires a manual operation by an operator, the guides may be urged forcibly or under pressure against the lateral edges of the sheets. This causes a loading on the sheets when the latter is to be fed, which may cause a failure to feed a sheet. When the guides are brought into registry with the scale, a skewing may result if a proper registration is not achieved. Additionally, some external forces may be unnoticeably applied to the sheets placed on the table or to the cross sheet guides to displace the sheet from a proper feeding position, again causing a skewing, a jamming or the failure to feed a sheet.
As a further consideration, an automatic sheet feeding apparatus of the bottom-fed type must be provided with some means which assures that the sheets in the stack will be subject to a friction exerted by the separating roller. In one form, such means may apply a pressure to the sheets of the stack from above, thus assuring the necessary friction. However, the magnitude of such friction or conveying force changes as the stack elevation varies, resulting in the disadvantage that an increased number of sheets cannot be placed in one stack while assuring a satisfactory separation and feeding.
In another known arrangement, the table is disposed at an angle as viewed in the sheet feeding direction so that the sheets placed in a stack thereon may slip over the table by gravity until their leading ends move into the nip between the separating roller and the brake member. However, when the table is disposed at an angle, the slip of the sheets in the sheet feeding direction may cause a concentration of sheets in the nip, inducing a failure to separate the sheets and increasing the tendency to produce an overlapping feed. On the other hand, when the table is disposed at a reduced angle, the friction acting between the separating roller and the sheets may be insufficient to assure a satisfactory feeding force, again resulting in the failure to feed a sheet. This occurs noticeably, in particular, when the separating roller also serves as a sheet feed roller. In addition, the smoothness or the weight of the sheets may vary depending upon the kind of sheets, and hence if the table is disposed at a given angle, it is very difficult to assure a feeding force of a given magnitude. If the angle of inclination of the table is adjustable, the arrangement cannot be easily changed to accommodate a stack having a mixture of different kinds of sheets. Another factor which further complicates the situation is the planarity of the sheets. Many sheets often contain wrinkles, a waviness, or an upward or downward warp, representing a departure from perfect planarity. Consequently, it is a rare occurrence that the leading end of the sheets in the stack will be brought into the nip between the separating roller and the brake member in a sequentially retrograde manner from the bottom or in alignment with each other. Rather, the stress in the sheets or the rigidness of the sheets may prevent the leading end of the sheets from reaching the nip, thus disabling the feeding operation or causing a fold or damage to be produced in the leading end if the feeding operation is attempted. If a fold is formed in the leading end of the sheet, it is very likely that a multiple feed results. Also, an apparatus of the bottom-fed type often has a problem in connection with the generation of static electricity, which may increase the tendency of inducing a multiple feed even if small in magnitude.
Accordingly, it is desirable that the sheet receiving table be disposed in a substantially horizontal position and that the separating roller be separate from the feed roller. When the table is disposed horizontally and a sheet feed roller is separately provided for feeding the leading end of sheets into the nip between the separating roller and the brake member, above disadvantages can be eliminated. However, the force of friction acting between the feed roller and the sheet which contacts it may become insufficient to assure a reliable feeding operation. To accommodate for this, vacuum means may be disposed below the table to attract the lower-most one of the sheets to the table, thus assuring a sufficient magnitude of the force of friction. However, since the feed roller has its peripheral surface partly projecting above the sheet receiving surface of the table, there is a likelihood that when sheets are placed on the table, the leading end or the end of the sheets which freely depends from the table may abut against the projecting part of the roller to be folded thereby. The underside of the sheets is not visible to an operator, and hence the folded sheets may be left uncorrected, thus causing a failure of a sheet feeding. The fact that the feed roller partly projects above the sheet receiving surface implies that there is a gap left between the sheets and the surface. Hence, when the vacuum means attracts the lowermost one of the sheets, the air may find its way through the gap, thus diminishing the attraction applied to the sheet. This results in an insufficient magnitude of the force of friction acting between the feed roller and the sheet. The purpose of applying the attraction by the vacuum means is to produce a force of friction acting between the feed roller and the sheet which is of a sufficient magnitude to convey a single sheet into the nip between the separating roller and the brake member, which are disposed in abutment against each other. If the force of friction has a reduced magnitude, a satisfactory sheet conveying cannot be assured. To provide a sufficient force of friction, the attraction applied by the vacuum means may be increased in magnitude. However, when sheets having a reduced rigidity are used, the attraction acting between the sheet receiving table and the sheet may exceed the force of friction, resulting in a failure to convey a sheet.
It is to be recognized that the sheets disposed in a stack do not always maintain their planarity, but frequently exhibit departures therefrom such as wrinkles, waviness, upward or downward warp. Hence, when the sheets are directly disposed in a stack without previously correcting their departure from the planarity, it is a rare occurrence that the leading ends of the sheets be disposed in the nip between the separating roller and the brake member in a sequentially staggered from or in alignment. Rather, the stresses in the sheets or the rigidity of the sheets may prevent the leading end of the sheet from reaching the nip to cause a failure to feed a sheet, or if the sheet is fed, the leading end of the sheet may be folded or damaged. Such difficulties occur independently from the orientation of the sheet receiving table.
Above difficulties can be elminated if an operator corrects for poor planarity of the sheets before the latter are placed on the table. However, the usual practice is often as simple as turning a folded edge back. Thus, it must be realized that the leading end of the sheets is seldom disposed in a separating station while maintaining an ideal planar condition, but that in practice, the sheets are fed under a condition which is often astable enough to cause an overlap feed or the failure to feed a sheet.
It should be noted that the materials for the separating roller and the brake member are chosen and their surface treatment made to provide a relationship given below, EQU .mu.2&gt;.mu.3&gt;.mu.1
where .mu.1 represents the coefficient of friction between sheets, .mu.2 between the separating roller and sheet and .mu.3 between the brake member and sheet.
What is contemplated by such relationship is to assure that only the lowermost one of the sheets be fed as a result of the force of friction (.mu.2) acting between the separating roller and the sheet exceed in magnitude the force of friction (.mu.3) acting between the brake member and the sheet. If the above inequality is satisfied, the magnitude of the coefficient .mu.2 is not much greater than that of the coefficients .mu.3 and .mu.1, but the inequality is maintained by a small difference among these magnitudes. When a sheet to be fed is high quality paper, the magnitude of .mu.3 is on the order of 0.8 to 0.9 and the magnitude of .mu.1 is on the order of 0.5 to 0.7 as referenced to the magnitude unity of .mu.2. Consequently, when the separating roller feeds a sheet by utilizing the force of friction (.mu.2), the force of friction (.mu.3) acting between the brake member and the sheet represents a loading on the separating roller which causes the sheet to be braked as it is being conveyed by the feed roller, causing a slip to occur between the separating roller and the sheet. In other words, the speed of the sheet being fed by the feed roller is different from the speed of the sheet. as the latter passes through the separating station. By way of example, in a facsimile system, originals to be transmitted comprise sheets having varying properties, and the speed of the sheet in the separating station is slower than the speed of the sheet being fed by the feed roller by an amount on the order of 20% to 30%. Since the sheet is being conveyed by the feed roller while its leading end is braked, it is possible that in worst cases, the leading end of the sheet may be folded, skewed or the image may be subjected to a rubbing action. When the feed roller is separately provided, a conventional design is that the feed roller functions to convey a length of sheet which is approximately equal to one circumference length of the roller. In other words, the design is such that one revolution of the feed roller is sufficient to convey the leading end of the sheet into the nip between conveying rollers which are disposed downstream, as viewed in the sheet feed direction. However, when the leading end of the sheet is braked in the manner mentioned above, the transmission of the drive to the feed roller is interrupted before the leading end of the sheet reaches the nip between the conveying rollers, In the event of such occurrence, the sheet must be conveyed by the separating roller, causing the failure to feed or a damage of the image as by rubbing action.
As a matter of practice, sheets having a variety of surface characteristics are used as originals which are to be transmitted in a facsimile system or to be copied in a copying machine. As a result, during a sheet feeding operation, an elastic material contained in the brake member may be subject to an oscillation as it slips depending on the variety of the sheets. Such oscillation of the elastic material may be transmitted through its support to other members. In particular, a strip-shaped member may be caused to vibrate in the manner of resonance, thus producing an uncomfortable sound. This phenomenon is attributable to the fact that the brake member is fixedly mounted. In addition to causing the sound, the fixing of the brake member may cause a biased abrasion of the elastic blade or a skewing of sheets depending on the manufacturing or assembly tolerances of parts in the separation unit.